1. Field of the Invention
This invention relates to a gas-cooled dynamoelectric machine, such as a turbogenerator, with a liquid cooled stator; and more particularly to an improved apparatus and method for detecting any leakage in the cooling system of said machine.
2. Description of the Prior Art
Effective cooling for turbogenerators has been obtained by utilizing a bifurcated cooling system. Various parts of the machine are cooled by circulating a pressurized gas coolant contained within the gas tight casing of the dynamoelectric machine. Still other parts of the machine are further cooled by a liquid coolant which is circulated in close heat transfer relation with the metal of the current carrying conductors; such as, with a coolant fluid passage disposed within the main ground insulation which electrically separates the copper conductor from the stator laminations. In this example the stator windings may be hollow, electrically conductive bars through which a water liquid coolant is circulated in a closed system. The gas coolant may be hydrogen gas.
Conventionally, the gas coolant within the frame of the machine is kept at a higher pressure than that of the liquid coolant used for the internally cooled stator windings. This pressure gradient between the two cooling systems makes it possible to detect a leak in the liquid system inside the generator. Should a leak develop, the gas enters the liquid coolant system rather than the liquid escaping from its conduit into other parts of the dynamoelectric machine. Leakage of liquid out of the hollow conductors cannot occur under normal operating pressures in such a coolant system due to the pressure gradient. It is therefore advisable to detect any increases in the presence of gas entrained in the liquid cooling system by a gas monitor and warning system.
U.S. Pat. No. 2,970,232, to Kilbourne, issued Jan. 31, 1961, discloses a pressurized liquid cooling system for a gas-cooled, liquid-cooled generator; that is, for a bifurcated cooling system similar to that discussed above. The Kilbourne patent generally recognizes that increasing the liquid coolant pressure will raise the boiling point of the liquid coolant in the system. However, the patent does not include a leak detection device.
In the aforementioned closed liquid cooling system, the boiling point of the liquid is raised by increasing the static pressure throughout the liquid coolant. This not only increases the heat removal capability of the coolant, it retards coolant boiling. Coolant boiling may block the liquid path through the conduit, resulting in further vaporization. One disadvantage of increasing the static pressure in the liquid cooling system is that it increases the possibility of failure in the liquid connector joints and hoses. The liquid should be pressurized sufficiently to increase the boiling point, while at the same time the liquid pressure must remain below that of the gas coolant.
Several leak detection devices have been patented. One type of detection system merely identifies the existence of the coolant gas in the liquid coolant reservoir.
U.S. Pat. No. 2,675,493, issued Apr. 13, 1954 to Grobel recognizes that a detection of the presence of gas in the liquid coolant, which is kept at a lower pressure than the gas coolant, indicates the presence of a leak in the coolant system.
U.S. Pat. No. 3,122,668, issued Feb. 25, 1964 to Cuny proposes another arrangement for indicating leakage between cooling systems of turbogenerators. Prior to this patent, it was known that the presence of hydrogen gas as an indication of a possible leak could be detected by withdrawing the gas from a suitable point along the cooling liquid circuit under vacuum and supplying it to a gas analyzer. Such an indicating system, however, was complicated and expensive due to the vacuum system. In the Cuny patent, the indication of leaks between the two cooling systems of the generator, i.e., between the hydrogen coolant and the oil (which is used as the liquid coolant), is effected by a manometer arranged to directly measure the pressure of the gas in the oil tank. As hydrogen gas penetrates into the coolant oil system, the gas pressure in the tank rises. The Cuny patent teaches that measurement of this pressure will indicate the existence of leakage.
Measurement of the pressure in the coolant reservoir is problematic. Since a certain amount of leakage can be expected during normal operation of the dynamoelectric machine, not all pressure increases indicate a developing serious condition. Furthermore, the earlier the warning of a developing serious condition, the more likely it can be minimized and corrected. A leakage warning system should therefore respond as quickly as possible to unexpected pressure increases. Unfortunately, this objective frequently is limited by the sensitivity of the detection system. The sensitivity, after all, must be such that it discounts the expected pressure increases.
A warning system which can inform the operator of an unhealthy situation developing in the generator may prevent serious leakage and allow for repair during planned outage such as during low power demand periods. Such a continuous monitoring system should be capable of sensitive metering, with rapid response time, of small, unexpected pressure changes in the coolant tank.